This invention relates to video signal processing, and more particularly to an improved system for transforming a video image in one two-dimensional plane into another two-dimensional plane according to an arbitrary mapping function.
In the field of live video special effects, it is desirable to be able to map one image into another with as much flexibility as possible. Observation of modern television gives an indication of many uses of video special effects, including spatial transformations such as rotations, tear-away images, folding and unfolding images, images moving at different angles to the plane of the actual picture, etc.
In a digital video production facility, a field of video, whether in composite or component form, is processed as a series of luminance, chrominance, keying and (possibly) depth values at sequential pixel locations. A field or frame of such video is transmitted one line of pixels at a time, starting at the top left of the field and moving across one line, then the next, etc. The resulting digital signal can thus be thought of as a two-dimensional real-valued function sampled on a rectangular grid.
It would be desirable to be able to receive a stream of data representing such a video image and map it from a source coordinate system (h,v) into a destination coordinate system (x,y). For simple linear mappings this is a trivial problem. However, for a mapping that is non-linear or many-to-one and therefore able to map an image even if the resulting image is distorted and folded over in a complicated way, a fast and effective transformation mechanism is much more difficult to attain.
An example of such a mapping system is described in U.S. Pat. Nos. 4,563,703, 4,709,393 and 4,757,384, all assigned to Quantel Limited. One aspect of this video signal processing system is the way it produces mapped addresses. As described in the '393 patent at column 5, line 29, to column 6, line 24, mapping shapes are generated by a computer and stored in a disc store. Because access to the disc store is slow relative to video speeds, address interpolation is used both spatially and temporally. (Temporal interpolation is interpolation between two video frames separated in time by other video frames.) Despite the inventor's statement that: "We have found that this technique does not produce any noticeable degradation to the picture produced.", certain types of pictures are adversely affected by these interpolation techniques.
Another aspect of the video signal processing system disclosed in the '393 patent is shown in FIGS. 3 and 4 of that patent and described in the text at column 3, line 6, to column 4, line 43. Four channels of multipliers, summers and framestores process an incoming picture point into a bank of four adjacent pixels in the output video frame, using a series of read-process-write operations. The contents of the same pixel in the four separate framestores are summed together at readout time. It would be desirable to be able to accumulate data for four adjacent output pixels without employing four complete framestore memories.
What is desired is a new approach to video image processing that more efficiently utilizes memory resources and which operates fast enough to process a video image stream without introducing artifacts through temporal and spatial interpolation.